Catalysts for the production of ethylene oxide from ethylene and molecular oxygen are generally supported silver catalysts. Such catalysts are typically promoted with alkali metals. The use of small amounts of the alkali metals potassium, rubidium and cesium were noted as useful promoters in supported silver catalysts in U.S. Pat. No. 3,962,136, issued June 8, 1976, and U.S. Pat. No. 4,010,115, issued Mar. 1, 1977. The use of other co-promoters, such as rhenium, or rhenium along with sulfur, molybdenum, tungsten and chromium is disclosed in U.S. Pat. No. 4,766,105, issued Aug. 23, 1988, and U.S. Pat. No. 4,808,738, issued Feb. 28, 1989. U.S. Pat. No. 4,908,343, issued Mar. 13, 1990, discloses a supported silver catalyst containing a mixture of a cesium salt and one or more alkali metal and alkaline earth metal salts.
The use of ceramic based catalyst carriers and specifically alpha alumina based catalyst carriers has been previously described in a number of patents such as, for example, U.S. Pat. No. 5,100,859, issued Mar. 31, 1992, U.S. Pat. No. 5,055,442, issued Oct. 8, 1991, U.S. Pat. No. 5,037,794, issued Aug. 6, 1991, and U.S. Pat. No. 4,874,739, issued Oct. 17, 1989. Such catalyst carriers have a wide variety of potential applications in the catalytic field and are especially useful where the ceramic base is an alumina such as alpha alumina.
A catalyst support needs to possess, in combination, at least a minimum surface area on which the catalytic component may be deposited, high water absorption and crush strength. The problem is that usually an increase in one can mean a reduction in another property. Thus, high crush strength may mean low porosity. Often the balance is achieved by trial and error making the catalyst carrier art even more unpredictable than other chemical process art.
Carriers based on alpha alumina have an excellent balance of crush strength, abrasion resistance, porosity and catalytic performance that makes them ideal for a range of catalytic applications. It has been found that the physical properties can be improved by incorporating a titania component into the mixture fired to produce the carrier. While such titania modification has been found to greatly improve physical properties such as crush strength and abrasion resistance, it has been found that it does tend to affect the densification of the carrier structure and this can lead to unacceptable properties. This problem increases with increasing concentration of added titania.
There is therefore a need to provide for the incorporation of the highly beneficial titania component into the carrier without causing such densification such that the catalysts prepared therefrom have acceptable physical properties and performance characteristics. The catalysts of the present invention which are prepared using these carriers have excellent physical properties and catalytic performance that make them ideal for a wide range of catalytic applications.